1. Field of the Invention
The present invention relates to a record mode discriminating circuit which is adopted in 8 mm video for recording operations in two modes, a standard mode and a long time mode.
2. Description of the Prior Art
Generally, the 8 mm video records pilot signals for tracking use on FM picture signals without formation of control tracks. At the reproduction, tracking error signals are formed according to the level ratio of the particular two beat components of an injunction pilot signal and a reproduction pilot signal. At the reproduction, normally the tracking error signal is fed to a capstan servo to cause the tape to run at a constant speed which is the same as that at the recording.
On the other hand, this 8 mm video has a standard record mode and a double long-time record mode. When the tape recorded in the standard record mode is reproduced in the long time mode, the tracking error signal changes at an 8 field period, when the tape recorded in the long time reproduction mode is reproduced in the standard reproduction mode by mistake, the tracking error signal varies at a 4 field period. A record speed discrimination circuit of a tape for use in this 8 mm video is disclosed in U.S. patent application Ser. No. 699,628, filed on Feb. 8, 1985 in the name of Sachio Yamashita and assigned to the same assignee, SANYO ELECTRIC CO., LTD.
When the period of the tracking error signal is counted, the tracking errror signal is desired to be rectangular in wave. Thus, although the waveform is desired to be shaped, with the threshold level as a border, by a comparator, it becomes difficult to set the threshold level because of the large variation in the tracking error level through variation in the reproduction output level, and furthermore, it becomes difficult to shape the correct waveforms because of production of a plurality of cross points, which are to be primarily one in number, between the threshold level and the tracking error signals when the rectilinearity of the record track is wrong.
Also, when the reproduction characteristics of the video head are taken into consideration, the reproduction level of the signal to be provided through the direct scanning operation of the track by the head gap has a characteristic of 6 dB/oct to be reproduction signal frequency and the reproduction pilot signal level becomes directly proportional to the reproduction pilot signal frequency. The reproduction level of a signal to be reproduced by the track disengaged from the head gap depends upon the signal frequency. Accordingly, even if the ideal record track is ideally scanned, the tracking error signal does not become 0 and vriation which has a given periodicity is assumed in dependence on the reproduction pilot signal frequency.
Furthermore, in the production of the actual 8 mm video, the head gap width is disengaged from the original value because of the inferior yield in the head production. Also, the mounting stage difference may be caused even in the head mounting operation. The tracking error signal varies even in accordance with the variation in such a manufacturing process, with the result that the head comes to scan the video track so that the average value of the tracking error signal becomes 0.
Hereinafter, variations in the tracking error signal will be examined through the variations in head gap width, head gap width, the various changes in head mounting stage difference or the like. FIG. 7 shows the tracking error signal waveforms of 4 field portions in a case where the record track in an ideal standard mode of 20.5 .mu.m in track pitch is reproduced in a standard mode. A first stage of signal waveform shows the tracking error signal form in a case where the pair of head gaps width is 24 .mu.m, second, third stages of signal waveforms show the tracking error signal waveforms when the pair of head gaps width which should be originally 24 .mu.m has become irregular to 23 .mu.m, 25 .mu.m, also furthermore, fourth, fifth stages of signal waveforms show the tracking error signal waveforms when the pair of head gaps width has been spread to 28 .mu.m, 38 .mu.m. In addition, the tracking error signal waveform of each stage is one when a second row has a stage difference of 1 .mu.m, a third row has a stage difference of 2 .mu.m in a case where a first row has no stage difference. It is to be noted that the tracking error signal waveform may be obtained by a known operation equation.
As apparent from this diagram, the tracking error signal waveform of 15 Hz is assumed when the head gaps is inferior, when the head has mounting stage difference. However, if such unnecessary variation of 15 Hz is caused in the tracking error signal when the record pattern of the standard mode is reproduced correctly in the standard mode, a discrimination error is caused as if a record mode discrimination circuit scanned the record track of a long time mode. Although it seems better to improve a discrimination circuit so that a discrimination output may be transmitted only when the tracking error signal changes in 15 Hz largely beyond the threshold level so as to prevent the discrimination error, it is difficult in fact to specify the threshold level, because the reproduction output level changes by about 10 dB because of the characteristics of the record medium and the record reproduction characteristics so that the variations width of the tracking error signal may change in accordance with the reproduction output level.